Oxidation is usually recognized as the major cause of development of a stale flavor in beer. However, no significant difference in trans-2-nonenal concentration has been observed between oxygen-receiving and oxygen-free beers after aging. Moreover,O- 18(2) was not incorporated into the carbonyl fraction, indicating that the cardboard flavor in beer is not due to lipid oxidation in the bottle. As shown by adding deuterated nonenol to the pitching wort, nonenal oxidation and sulfitic adduct degradation were also inefficient pathways of trans-2-nonenal synthesis. On the other hand, wort nonenal linked to amino acids and proteins revealed to be able to release nonenal at the beer pH. The measured nonenal potential proved a good indicator of beer staling; therefore, UV spectroscopy was used to find which experimental conditions destabilize trans-2-nonenal precursors in wort Laboratory-scale experiments confirmed that SO2 can reduce both lipid autooxidation and the nonenal potential rise while the wort is boiling.